A polybutylene terephthalate resin is used as an engineering plastic in a wide range of applications such as automobile parts and electric and electronic parts since it is excellent in mechanical properties, electrical properties and other physical and chemical properties, and satisfactory processability.
Among the above applications, there exists a case which accommodates a substrate or the like with electronic parts mounted thereon, and this case is obtained by bonding a lid with a body. Such case is used to protect parts such as a substrate accommodated inside the case from moisture, dust, and impact from the outside, thus requiring sufficient adhesion between the lid and the body.
There has been known, as a bonding method which can enhance bonding strength between the lid and the body, and is also simple and easy, welding methods such as a vibration welding method and an ultrasonic welding method. The above welding method is preferably employed for bonding the lid with the body in the case as mentioned above, and is also preferably employed for bonding molded products, each being composed of not only the polybutylene terephthalate resin but also other resins, together.
For example, Patent Document 1 discloses the technology for bonding a lamp housing made of a thermoplastic resin composition with a lens made of a polymethyl methacrylate resin or a polycarbonate resin using a vibration welding method to produce an automobile lamp. As mentioned above, the technology for bonding molded products together using a welding method has hitherto been known. However, the mechanical strength of a welded body must be taken into consideration in actual product development. In Patent Document 1, a raw material composing the lamp housing is a rubbery graft copolymer obtained by polymerizing a methyl methacrylate monomer with an ethyl acrylate monomer, or a resin composition containing a polycarbonate resin and a polyalkylene terephthalate resin. This resin composition has low rigidity and is inferior in heat resistance such as deflection temperature under load. Therefore, even if a case accommodating a substrate is produced using the technology disclosed in Patent Document 1, a problem of mechanical strength arises, thus failing to obtain a case having desired performances. Patent Document 1 discloses only appearance (melting width) as vibration weldability, and does not make a suggestion about the weld strength.
Patent Document 2 discloses a polybutylene terephthalate resin composition for the production of a molded product to be subjected to vibration welding. The resin composition disclosed in Patent Document 2 contains a modified polyester copolymer containing 5 to 30 mol % of a comonomer. Since heat resistance of the molded product deteriorates when using this modified polyester copolymer, it is impossible to obtain a case having desired performances even if a case accommodating a substrate is produced using the technology disclosed in Patent Document 2.
Patent Document 3 discloses a polyamide resin composition for the production of a molded product to be subjected to vibration welding. Patent Document 3 discloses that the viscosity upon melting of resin composition may be increased so as enhance weld strength between molded products.
[Patent Document 1] Japanese Unexamined Patent Application, Publication No.2000-302824
[Patent Document 2] Japanese Unexamined Patent Application, Publication No.2006-176691
[Patent Document 3] Japanese Unexamined Patent Application, Publication No.2009-97011